The present invention relates to an image pickup device using a solid-state imaging device and a driving controlling method for the solid-state imaging device. More specifically, this invention relates to an image pickup device realizing the zoom function for enlarging a portion of an image picked up with a solid-state imaging device and a driving controlling method for the solid-state imaging device.
Recently a digital still camera or a digital video camera using therein a solid-state imaging device such as a CCD (Charge Coupled Device) has been popularized, and most of the image pickup devices as described above have the zoom function. As a method of realizing the zoom function, there have been known a method of using an optical lens moving mechanism and another method not using the lens moving mechanism, and in a method categorized as belonging to the latter type, a zoom ratio of N times is achieved with a solid-state imaging device based on the interline transfer system by intermittently transferring a signal charge for each pixel in an area to be enlarged within an acceptance surface once for N horizontal scan periods (N·1) (Refer to, for instance, Japanese Patent Laid-Open No. hei 1-157678 (page 5 to page 9, FIGS. 1 to 4)).
FIG. 9 is a timing chart showing timing for driving a CCD when the method described above is employed.
In a CCD based on the interline transfer system, a number of photoreceptor sections each carrying out photoelectric conversion are arranged in the matrix form, and a vertical transfer register is positioned to each of the photoreceptor sections arrayed in a row, and each vertical transfer register is connected to one horizontal transfer register. Signal charges transferred from the photoreceptor sections to corresponding vertical transfer registers are successively transferred by the horizontal transfer registers line by line.
In order to realize the N times zoom function with the CCD and without using any optical means, a portion of a central area of a photoreceptor surface is used as an enlarged area, and the CCD is driven at the timing as shown in FIG. 9 according to a horizontal drive (HD) signal and a vertical drive (VD) signal. Namely, in a vertical blanking period, signal charges for all photoreceptor sections are drained (timing T301 to T302), and then signal charges for an unnecessary area in the upper edge section of the photoreceptor surface of the CCD are transferred at a high speed for discharging (timing T302 to T303). Next, signal charges for a necessary area in the vertical direction are intermittently transferred at a speed of 1/N against the horizontal synchronism in a vertical effective period starting from the timing T304. In the vertical effective period, unnecessary charges in the right and left edge sections of the photoreceptor surface are aborted. With the image signals outputted according to the drive timing as described above, an image corresponding to the enlarged area on the photoreceptor surface is magnified by N times, so that a zoom image magnified N times can be obtained without using any optical means.
With the solid-state imaging device based on the interline transfer system, however, when a potential gap is present in a charge transfer area of a vertical transfer register, a pixel corresponding to this position disadvantageously becomes a defective pixel (black point). To solve the problem as described above, there has been proposed a solid-state image pickup device in which an electric charge can be injected from a drain area of a vertical transfer register to a vertical transfer register to drain unnecessary charges to the drain area, then the charge injected from the drain area is transferred to a horizontal transfer register, and then a charge comprising signal charges from the photoreceptor section is read into the vertical transfer register (Refer to, for instance, Japanese Patent Laid-Open No. Sho 63-257388 (page 505, FIG. 7)). Further, there has been used more commonly the method in which a structure of a solid-state imaging device is not changed as described above, and also in which a position of a defective pixel is previously stored in a memory and a signal from the defective pixel included in an output image signal from the solid-state imaging device is interpolated with signals from pixels around the defective pixel.
When the zoom function is realized by driving a CCD as shown in FIG. 9, the vertical blanking period and a vertical effective period are fixed according to an output frame rate for am image pickup signal (for instance 30 frames/sec), and further a transfer frequency for high speed transfer of signal charges from an unnecessary area is basically kept constant, and therefore when the zoom ratio is, for instance, 1.2 and the number of lines to be drained by means of high speed transfer, a transfer suspended period generated in the time frame from timing T303 to T304 becomes longer. During this transfer suspended period, signal charges are accumulated in a vertical transfer register, so that, when the defect due to a potential gap as described above is present in the vertical transfer register, a pixel corresponding to the defective position becomes a defective pixel.
Because the number of lines transferred at a high speed in the time frame from timing T302 to timing T303 changes according to a zoom ratio, positions of defective pixels generated in the entire image during the transfer suspended period change according to the zoom ratio. Because of this problem, positions of the defective pixels can not previously be stored in a memory, and the method of interpolating each of the defective pixels with signals from pixels around the pixel can not be employed, which is disadvantageous.